Human activity linked to rise in hurricanes

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Human-induced climate change, rather than naturally occurring ocean cycles, may be responsible for the recent increases in the frequency and strength of North Atlantic hurricanes, according to MIT and Penn State researchers.

"Anthropogenic factors are likely responsible for long-term trends in tropical Atlantic warmth and tropical cyclone activity," the researchers report in an upcoming issue of Eos, the weekly newspaper of the American Geophysical Union.

The Atlantic hurricane season begins June 1.

From data going back to the 19th century, scientists have found a correlation between the temperature of the sea surface in the tropical Atlantic and tropical cyclone activity. Warmer surface temperatures are associated with increases in strength and duration of cyclones.

But what is causing the increased surface temperatures?

Some scientists believe human-induced climate change is behind the trend. Others cite a natural cause, the so-called Atlantic Multidecadal Oscillation (AMO) -- an ocean cycle similar to, but weaker and less frequent than the El NiÌ±o/La NiÌ±a cycle.

Kerry A. Emanuel, professor of atmospheric sciences at MIT, and Michael E. Mann, associate professor of meteorology and geosciences at Penn State, used a statistical method to separate the influences of one from the other.

"The important result of this work is that the tropical North Atlantic sea surface temperature appears to be controlled largely by radiative forcing, which has changed over the past century mainly owing to sulfate aerosol pollution and greenhouse gas increases," Emanuel said.

"There is no evidence of any 'natural cycle' in tropical North Atlantic sea surface temperatures in late summer and early fall. And since the sea surface temperature is strongly linked to Atlantic hurricane activity, this suggests there is no 'cycle' in the latter," he said.

In a seeming paradox, Emanuel and Mann also found that some pollutants have actually mitigated the warming problem.

Some gases, such as carbon dioxide and methane in the upper atmosphere, create the greenhouse effect associated with global warming; other pollutants, such as sulfur dioxide and nitrogen oxides in the lower atmosphere, cool the Earth's surface by reflecting sunlight.

Because of prevailing winds and air currents, pollutants from North America and Europe move into the area above the tropical Atlantic. The impact is greatest during the late summer when the reflection of sunlight by these pollutants is greatest, exactly at the time of highest hurricane activity.

This suggests that the cooling from pollutants in the atmosphere tempered the rise of sea surface temperatures and number of hurricanes. However, the industrialized world is doing much better at controlling the amounts of aerosols going into the atmosphere, and the cooling effect has been decreasing since the 1980s.